Press Sauté—normal/medium on the Instant Pot and heat the oil. Add the onion, garlic, serrano chiles, cumin, and pepper and cook, stirring constantly, for about 1 minute, until slightly softened. Add the broth and flour and stir until well combined. Press Cancel. Add the cream cheese, Cheddar cheese, Jack cheese, and tomato and stir until the cheeses melt.
Secure the lid and set the Pressure Release to Sealing. Press Pressure Cook, then set the cooking time for 1 minute.
When the cooking program is complete, press Cancel. Perform a quick pressure release by moving the Pressure Release to Venting. Open the pot, then whisk the queso until smooth. Press Warm and serve directly from the Instant Pot, or transfer to a warmed serving bowl. Serve with tostadas and flour tortillas for dipping.